MMB Breaks in from orthographic view to 3d view
I am assuming you are using Blender 2.8. In 2.8, pressing 1, 3, and 7, locks you view to X, Y, and Z axis, as well as setting your view to orthographic. Breaking this view disables orthographic mode. If you wish to enable it manually, and view other angles, press 5 on the numpad.